This invention relates to apparatus for generating analog composite video signals and, more particularly, to apparatus for generating a wide range of said signals.
Analog composite video signals have several amplitude levels, i.e.: a white (maximum video) level; a black (minimum video) level; a blank level; and a sync level. When generating analog composite video signals, a problem is encountered in that different video displays have different level requirements, often having close amplitude tolerance specifications. For example, one monitor arrangement accepts a compatible composite video signal requiring particular white and sync levels whereas another compatible monitor arrangement requires other white and sync levels. Non-standard monitors require a still different set of signal levels. In some cases, the signal levels can be three to five times higher than ordinary standards. Accordingly, video signal generation must be accomplished so as to provide different signal levels with sufficient resolution to meet particular requirements.
Conventionally, digital video data are converted to analog composite video signals by using high speed video digital to analog converters to produce the required signal levels. Different signal levels require the use of ultra-high resolution converters. In this regard, it will be understood that the converters are used to control the highest video white level in order to have acceptable resolution capability at lower levels. This "brute force" approach places the burden on component manufacturers to provide precision, high speed, high resolution video digital to analog converters to meet the resolution requirements of both high and low white level video signals. This approach is disadvantageous because the availability of the required digital to analog converters is limited and their cost is prohibitively expensive. Further, digital to analog converters of the type described are difficult to use because their noise sensitivity increases with their resolution, requiring careful circuit design resulting in increased effort and cost.
The present invention reduces the number of high speed digital to analog converters from that otherwise necessary and eliminates the need for ultra-high resolution converters, while maintaining the highest possible full scale digital to analog converter resolution for all white video levels. This arrangement, based on a substantially consistent ratio between the white, sync and black signal levels, results in both component and labor cost savings without compromising the ultimate result.